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Venus Returns for Its Shining Hour

The world is about to witness a rare spectacle that once launched expeditions to ideal vantage points around the globe and inspired millions of people to venture outside and stare at the heavens.

On June 8, people in the right places on Earth will be able to see Venus move across the face of the Sun in a kind of minieclipse that is visible twice every century or so. The last such occurrence, called a transit of Venus, was in 1882. It inspired an international effort to use the event to answer one of the most pressing scientific questions of the day: What is the exact distance between the Sun and Earth?

Although studies of the event failed to provide an exact answer, they did narrow the range of estimates and measurements, and ushered in an era of investing in science as a symbol of national prestige. For the last event, the United States government mustered eight expeditions to make observations around the world, partly because Britain, France, Russia and other rivals did the same.

By bouncing radar signals off the Sun and Venus and using spacecraft measurements, scientists in the 1960's calculated that the average Sun-to-Earth distance is 92,955,859 miles, a measure called the astronomical unit.

Scientists realized for centuries that if they could find out that number, they could use the formulas of the 17th-century astronomer Johannes Kepler to calculate the size of the solar system and the exact distances between the planets.

''This was the most important question of its day in astronomy,'' said Dr. Jay M. Pasachoff, a professor of astronomy at Williams College. ''And using the transits of Venus to calculate the astronomical unit was the best way to do it.''

Although transits of Venus have occurred for thousands of years, the first report of its subtle crossing of the Sun was in 1639. The transits occur when the orbits of Venus, Earth and the Sun put them into alignment along the same plane.

Since 1639, transits have occurred in 1761, 1769, 1874 and 1882. If someone misses the one next month, there will be another opportunity on June 6, 2012. After that, more than a century will pass before the next transits, in 2117 and 2125. Because of its rarity, the transit next month, best viewed from Europe and the Mideast, is generating great scientific and public interest, said Dr. Steven J. Dick, chief historian for the National Aeronautics and Space Administration. Dr. Dick has written extensively on the 18th- and 19th-century transits.

No one alive today saw the last transit, he said, and seeing the next two will be the only chance most people have.

''These are truly once-in-a-lifetime events,'' Dr. Dick said. ''Although the scientific importance has diminished, I think there will be a lot of interest this time among the public, based on e-mail I've seen from around the world.''

Dr. David DeVorkin, curator of the history of astronomy at the National Air and Space Museum, said the 1874 and 1882 transits were prominently featured in newspapers and magazines. A carnival atmosphere pervaded Wall Street for the transit on Dec. 6, 1882, with people crowding the area and staring up through smoked glass.

''It was a popular diversion,'' Dr. DeVorkin said. ''Something maybe everybody didn't try to see, but everybody talked about it.''

Scientific interest persists. Instruments aboard at least three Sun-watching satellites, as well as ground telescopes, will follow the event. Researchers will use Venus' trek to test techniques and instruments that can be used to detect planets in other solar systems.

More than 120 extrasolar planets have been discovered orbiting other stars, most of them huge bodies found because their gravity affected the motion of their stars.

Astronomers have recently detected a small number of far planets by measuring the fluctuations that they cause in light from the stars they circle. In 2007, NASA plans to launch the Kepler spacecraft to monitor Sun-like stars in hope of detecting Earth-size planets through small decreases in star brightness.

Although denied a direct view of the transit because it occurs at night in the American West, astronomers with the University of Arizona hope to get an indirect view. Dr. Glenn H. Schneider said he and a colleague, Paul S. Smith, would try to use the Steward Observatory in Tucson to measure about a half-hour of sunlight from the end of the transit as it reflects off the Moon.

''We want to see if we can detect the signature of Venus' atmosphere spectroscopically from sunlight reflecting off the moon, as if it was a reading coming from a faraway star,'' Dr. Schneider said.

The transits generally occur in a predictable pattern of two occurring in an eight-year period, followed by one 105 1/2 years later and another eight years after that. After an additional 121 1/2 years, the pattern repeats. The paired eight-year sightings occur because a Venusian year equals 224.7 Earth days, making 13 Venusian years equal to eight Earth years.

That allows the planets to return to about the same alignment with the Sun they had been in eight years earlier, after which they go out of sync for more than a century.

On Tuesday, June 8, observers lucky enough to view the entire transit will see Venus as a small black spot crossing the southern hemisphere of the Sun from left to right. The planet, entering the disc of the Sun at the 8 o'clock position, will take six hours to cross the bright face before exiting at the 5 o'clock position.

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Location is everything, particularly when trying to witness celestial events. The entire transit will be visible in Europe, most of Africa, the Mideast and most of Asia. The unlucky regions of the globe where the event occurs at night, and is unviewable, include western North America, including most of the United States west of the Rockies; southern Chile and Argentina; Hawaii; and New Zealand.

Some regions will see just part of the transit, because the Sun sets while it is in progress. Those areas include Australia, Indonesia, Japan, the Philippines, Korea and Southeast Asia.

Likewise, the Sun rises with the transit in progress over eastern North America, the Caribbean, western Africa and most of South America, allowing observers a brief view before the event ends. How much early risers see will depend on the weather and how high the Sun rises above the horizons before Venus moves out of view.

In New York, sunrise will be at 5:25 a.m., and Venus is to begin exiting the solar disc at 7:06, when the Sun is 17 degrees above the horizon. The planet's final contact with the edge of the Sun should occur at 7:26 a.m., when the Sun is 20 degrees high. Times are similar for most cities in the Eastern time zone and one hour earlier in the Central time zone. But moving West means that the Sun is lower on the horizon.

Modern interest in planetary transits can be traced from Kepler. Based on his calculations of planetary motion, he wrote in 1627 that Mercury would cross the face of the Sun in November 1631 and that Venus would follow on Dec. 6 that year. Kepler suggested that observers placed at widely different points on Earth could indirectly calculate the distance to the Sun by using Venus.

Knowing the distance between observers and the different angles from which they viewed the transit, astronomers could calculate the distance to Venus and use that to compute the Earth-to-Sun measurement, he reasoned.

Kepler died the year before the 1631 Venus transit, but he would not have seen it had he lived, because it occurred at night in Europe. He would have also missed the next transit, in 1639, because he made a miscalculation that failed to predict it.

Fortunately, a young English astronomer, Jeremiah Horrocks, became interested in Kepler's work and, in recalculating some of the German's tables, discovered that a transit would occur on Nov. 24, 1639. Horrocks witnessed part of the transit from his home in Much Hoole, Lancashire, and a friend whom he notified by letter, William Crabtree, saw it from Manchester.

The next transits, in the 18th century, drew much more attention, thanks to Edmond Halley, the British astronomer best known for the comet that bears his name. Halley suggested using the 1761 and 1769 transits to calculate the Sun-to-Earth distance by having observers time the events from widely spaced latitudes and trace the planet's path across the Sun's face as they saw it from their positions. By measuring the angular shifts of the paths based on the timings, Halley reasoned, the astronomical unit could be calculated.

Although Halley died in 1742, his plan guided many observations made of the two transits from around the world. But the results varied widely and were disappointing. Among those trying to work on the problem in 1769 was the British explorer Capt. James Cook, who took his ship, the Endeavour, on its first voyage to the South Pacific to observe the transit from Tahiti.

Cook and others were frustrated in their observations by the inability to time the exact moment when the edges of the planet and the Sun appeared to touch. When Venus nears the edge of the disc of the Sun, its black circle appears to ooze toward the edge of the sun without showing a clear point of contact. Although the precise second of contact was needed for calculations, this so-called ''black drop'' phenomenon caused observers watching the same event to disagree by several seconds up to a minute on when the outer edges touched.

Cook and other observers speculated that the problem was the distortion of light through the Venusian atmosphere.

Earlier this year, using spacecraft observations, Dr. Pasachoff and other scientists concluded that the black drop effect was caused by a combination of images' blurring in small-aperture telescopes and the natural dimming of sunlight near the Sun's visible edge.

In the 19th-century transits, scientists tried to overcome that effect and other imperfections with better telescopes and the introduction of photography. Still measuring and timing transits never led to finding the precise Sun-to-Earth distance.

William Harkness of the United States Naval Observatory refined results from the 1882 transits and in 1894 came up with an astronomical unit measure of 92,797,000 miles. But the work of another Naval Observatory scientist, Simon Newcomb, was adopted as the world standard at a 1896 meeting in Paris, Dr. Dick said. Newcomb, who gave little credence to transit data, combined values from several sources including speed-of-light star readings, to come up with a figure of about 92,872,000 miles. Both were close to the modern value of 92,955,859 miles, but precision is critical in astronomical terms.

Nevertheless, Dr. Dick said, the transits of Venus remain important because the desire to define the astronomical unit -- and to maintain or gain scientific prestige -- led many nations to mount competing expeditions. In 1874, Russian sent out 26 expeditions, Britain 12, the United States 8, Germany and France 6 each, Italy 3 and the Netherlands 1.

''You could compare it with the space race in the 20th century,'' he said.

How to Watch Without Harm

When viewing solar events like eclipses or the transit of Venus, precautions are needed. Never look directly at the Sun. The direct gaze can lead to severe eye damage or blindness, experts say.

Sunglasses and clouds do not protect the eyes, and viewing the Sun through unfiltered telescopes, binoculars or cameras can result in instant and permanent damage. Telescopes and binoculars should be equipped with special undamaged solar filters. Glasses with solar lenses are available commercially, but even then do not stare at the Sun for long periods.

The transit of Venus can safely be seen if viewed indirectly, using telescopes or pinhole boxes to focus the image on a screen or paper opposite the Sun.

Information on viewing solar events is at these Web sites:

www.transitofvenus.org/safety.htm

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html

More Web resources can be found at: nytimes.com/science. Warren E. Leary

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A version of this article appears in print on May 18, 2004, on Page F00001 of the National edition with the headline: Venus Returns for Its Shining Hour. Order Reprints|Today's Paper|Subscribe